1. Field of the Invention
The present invention relates to forming a metal silicide film, and more particularly, to a method of depositing a metal silicide film that is robust to stress cracks in subsequent thermal processing.
2. Description of the Related Art
Refractory metal silicides are attractive materials for semiconductor fabrication because of their high melting points and low resistivities. In particular, these materials are popular for use in the formation of integrated circuits as intermediate barriers or conducting films, runners or wiring. Metal suicides are also used for a polycide gate structure composed of a polysilicon layer and a metal silicide layer used for the gate electrode.
Metal suicides are typically formed by a deposition process such as chemical vapor deposition (CVD) or sputtering. For example, a forming tungsten silicide by a conventional CVD process involves the reaction between silane (SiH4) and tungsten hexafluoride (WF6). In order to avoid gas phase nucleation of the metal silicide on the walls of the reaction chamber, the ratio of SiH4 to WF6 is kept high, on the order of about 50:1. Tungsten silicide formed by this reaction is non-stoichiometric WSix, with the xe2x80x9cxxe2x80x9d factor depending on the silane flows and generally having a value of between about 2 and 3.
One problem with the deposition of metal silicides is that the resulting film often has high stress. For example, a tungsten silicide film formed by the conventional CVD process described above will have high residual tensile stress. Because such films are deposited at elevated temperatures, upon subsequent cooling of the film, a tensile stress is created by the differences in thermal expansion coefficients between the deposited film and an underlying substrate. Furthermore, stresses also arise in the deposited material due to defects such as dislocations in the resulting crystal structure. Because of the residual stress of the material, underlying topography and thermal processing of the device after WSix deposition makes WSix interconnects susceptible to stress cracks and material failure. While it has been found that increasing silane flows can reduce this stress, increasing silane results in poor step coverage for the film.
Accordingly, what is needed is a metal silicide film for a semiconductor device having low internal stress. In particular, this metal silicide film should be robust to subsequent thermal processing to prevent the formation of stress cracks in the film that may lead to material failure.
Briefly stated, a metal silicide film and method of forming the same are provided. The method comprises depositing metal silicide layers onto a substrate assembly with alternating layers of silicon. The resulting metal silicide film has a disrupted grain structure and smaller grain sizes than prior art films of the same thickness, which increases the resistance of the material to stress cracks in subsequent thermal processing and reduces the overall residual stress of the material.
In one aspect of the present invention, a method is provided for forming an electrically conductive film. First, a metal silicide layer is formed over a substrate assembly. Second, a silicon layer is formed over the metal silicide layer. Third, a metal silicide layer is formed over the silicon layer. Fourth, the steps of forming a silicon layer and forming a metal silicide layer over the silicon layer are sequentially repeated until a film of desired thickness is formed. In one preferred embodiment, these steps are repeated twice to form a film having a thickness of about 600 xc3x85 with each metal silicide layer having a thickness of about 150 xc3x85 and each silicon layer having a thickness of about one or two monolayers.
In another aspect of the present invention, a method of forming a tungsten silicide film by chemical vapor deposition (CVD) is provided. This method generally comprises the following steps:
(a) introducing a silicon gas flow into a CVD chamber;
(b) introducing a tungsten gas flow into the chamber which reacts with the silicon gas flow to form a layer of tungsten silicide;
(c) shutting off the tungsten gas flow from the chamber while maintaining the silicon gas source into the chamber;
(d) reintroducing the tungsten gas flow into the chamber after the step of shutting off, wherein the tungsten gas flow reacts with the silicon gas flow to form an additional layer of tungsten silicide; and
(e) sequentially repeating the steps of shutting off and reintroducing the tungsten gas flow n times, where n is an integer of 0 or greater.
In another aspect of the present invention, an electrically conductive runner or wiring in a semiconductor device is provided. This runner or wiring comprises alternating layers of metal silicide and silicon.
In another aspect of the present invention, a method of forming an electrically conductive runner or wiring in a semiconductor device is provided. This method comprises maintaining a silicon gas flow into a CVD chamber, and intermittently pulsing on and off a metal gas flow into the chamber to form a plurality of distinct metal silicide layers.
In another aspect of the present invention, a method of forming an electrically conductive film is provided. This method comprises depositing a plurality of layers of metal silicide, and depositing an interrupting layer between each layer of metal silicide.
In another aspect of the present invention, an electrically conductive film of total thickness t is provided. The film comprises a plurality of metal silicide layers and a plurality of interrupting layers, each interrupting layer being formed between layers of metal silicide. The average grain size in the metal silicide layers is less than about the total thickness t of the film divided by the number of metal silicide layers.
In another aspect of the present invention, a method of forming an electrically conductive film is provided. The method comprises forming a first silicide layer of a first thickness on a substrate so that the maximum vertical grain size is limited to the first thickness. The vertical grain growth of the first silicide layer is interrupted by forming an interrupting layer on an exposed surface of the first silicide layer. A second silicide layer is formed on an exposed surface of the interrupting layer. The step of interrupting the vertical grain growth in the first layer results in the electrically conductive film having reduced residual stress.